Multiplex communication system



Feb. 27, 1940.

F. c. P. HENROTEAU 2,191,565

IULTIPLEX COMMUNICATION SYSTEM Filed Jan. '15, 1938 2 Sheets-Sheet 1 dim F. c. P. HENRQTEAU 2,191,565

)IULTIPLEX COMMUNICQTION SYSTEM Filed Jan. 15, 1938 2 Sheets-Sheet 2 Patented Feb. 27, 1940 UNITED STATES PATENT OFFICE ginm Application January 15, 1938, Serial No. 185,224

In Belgium January 16, 1937 8 Claims.

This invention relates to an apparatus for transmitting a number of contemporaneous messages through a single transmission channel, 1. e., over the same wire or wires or on the same wave length. The expression "message is to be understood in a general sense as including any intelligence or any portion of any intelligence, such as sound or pictures or writing, which may be represented by electrical vibrations.

It has previously been proposed to transmit a number of contemporaneous messages through'a single transmission channel by recording the difierent messages simultaneously as variations in the magnetism of difierent steel tapes, which are moved simultaneously through different equal arcs of a common circle, about the centre of which an electromagnet. revolves in the opposite direction at a speed such that it travels round the whole circumference while any point on a tape travels over its arc. At the receiver is a similar arrangement. When the magnet at the transmitter moves past a portion of tape on which a message is recorded, it transmits signals to the synchronously moving magnet at the receiver which, in turn, records them on a corresponding tape moving in synchronism with the tape at the transmitter. The message is then reproduced from the latter tape. If the arcs at transmitter and receiver through which the different tapes 80 move are, for example, 36, the rotating electromagnet will move ten times as fast as the tape and ten contemporaneous messages may be transmitted. This device, which so far as is known has never been put in practical operation, 85 sufiers from the disadvantage of having a large number of mechanically moved parts, all of whose movements must be exactly synchronised if it is .to operate at all. There is, moreover, a definite limit to the number of contemporaneous mes- 40 sages which could be transmitted by it, as the cores of the electromagnets oppose the passage of high frequency currents, owing to the phenomenon of hysteresis.

According to the present invention mechanically moving parts are substantially eliminated and may be entirely eliminated, the messages being recorded as electrostatic charges which are translated into signalling impulses, so that no difficulty analogous to that of hysteresis arises when the number of contemporaneous messages to be transmitted is large. The apparatus of the invention consists essentially in a mosaic composed of mutually isolated conducting elements series of electrostatic charges on said mosaic, and means for scanning said mosaic in a time equal to that elapsing during the recording of the messages to translate the charges on said mosaic successively into signalling impulse. The record- 5 ing and translation may be effected simultaneously or alternately. In the latter case distinct electrostatic images of the diiierent messages are formed simultaneously. At the receiver the procedure may be similar to that at the transmitter 10 with the relation of speeds reversed, the signalling impulses received being recorded successively as distinct series of electrostatic charges for diflerent messages, at the same speed as the translation, and the messages being reproduced simultaneously from these last series of charges at the same speed as that at which they were originally recorded. If recording and translation were effected alternately at the transmitter, recording and reproduction are efiected alternately at the receiver and in. this case the received impulses are translated successively into distinct electrostatic images.

More specifically the apparatus may comprise at the transmitter a group of cathode ray tubes (the term cathode ray tube meaning, wherever it occurs in this application, a tube having a fluorescent screen scanned by an electron beam), a cell containing a mosaic composed of a multiplicity of mutually isolated photoelectric elements, means for focusing images of the fluorescent screens on the mosaic, means for controlling the intensity of the electron beam of each tube in accordance with a difierent message, means for scanning simultaneously in all the tubes, and means for scanning the mosaic to produce signalling impulses in a time equal to that elapsing during scanning in the tubes. At the receiver the apparatus comprises a receiving cathode ray tube, a number of receiving cells corresponding to the number of transmitting cathode ray tubes and each containing a mosaic composed of mutually isolated photoelectric elements, means for focusing difierent equal sized portions of the fluorescent screen of the receiv- 5 ing cathode ray tube on difierent mosaics, means for controlling the intensity of the electron beam in the receiving cathode ray tube in accordance with the signalling impulses received, means for causing this beam to scan the fluorescent screen in synchronism with the scanning in the transmitting cell, and means for scanning the mosaics in the receiving cells simultaneously in synchronism with the scanning'in the transmitting cathode ray tubes.

The invention will be more fully described in connection with the attached drawings, in which Figure l is a diagrammatic illustration of a transmitter,

Figure 2 is a diagrammatic illustration of an arrangement for permitting alternate recording of the messages and translation of the recorded charges into signalling impulses,

Figure 3 is a diagrammatic illustration of a receiver,

Figure 4 is a diagram of an arrangement which may be resorted to for utilizing the transmission channel during the whole time of transmission, and

Figure is a diagram of an arrangement which may be used in connection with the arrangement of Figure 4.

The transmitter of Figure 1 comprises a number of cathode ray tubes 2, 2', 2", etc., which are made as thin as possible in one direction (vertically in the drawings) and are arranged in a bank one on top of the other with their fluorescent screens 3, 3', 3", etc., in the same plane, a cell 4 containing a mosaic 5 of mutually isolated photoelectric elements carried on an insulating layer 6 formed on a conducting signal plate I, and an appropriate optical system 8 for focusing images of the fluorescent screens 3, 3, etc., on the mosaic. Each cathode ray tube and the cell is provided with the usual hot cathode 9 and accelerating anode H] for forming the electron beam, and pairs of deflecting plates II and I2 for causing it to scan the screen or mosaic as the case may be, the pair ll giving the horizontal scanning component and the pair l2 the vertical scanning component. Each cathode ray tube contains a control electrode l3 supplied with electrical oscillations corresponding to a difl'erent message and thus varying the intensity of the electron beam in that tube in accordance with such message. There may be as many cathode ray tubes opposite a single cell as desired or as it is possible to put into a space such that images of all their fluorescent screens may be formed simultaneously on the mosaic of the cell; a small number has been shown in the drawings simply to make the description easier.

The electron beam in each cathode ray tube, varying in intensity in accordance with a given message, determines the production on the fluorescent screen of the tube of a series of light spots similarly varying in intensity, and the light spots on all the screens are impressed simultaneously on the mosaic 5 as they are formed, to produce electrostatic charges thereon. When one scanning has been completed in the cathode ray tubes, the messages supplied to these tubes in that time, which in probably all cases will be mere fragments of complete conversations or other series of sounds or of complete images, will have been recorded on the mosaic 5 as distinct series of electrostatic charges. In a time equal to that elapsing during one complete scanning in the tubes 2, 2, etc., the mosaic 5 is completely scanned by the electron beam in the cell 4 and any charges formed on elements of the mosaic before such elements are struck by this beam will be translated into signalling impulses in the manner well known.

Recording of the messages, i. e., scanning in the tubes, may take place at the same time as translation into signalling impulses of electrostatic charges formed, i. e., scanning in the cell, but it may sometimes be found undesirable to allow the charging current resulting from the recording to be transmitted. In this case scanningin the tubes and scanning in the cell will be eflected alternately, for example by means of the arrangementshown in Figure 2. This arrangement involves the use of two identical sets of transmitting devices, that is of an extra group of cathode ray tubes 2a, 2a, etc., only one of each group being shown in the figure, and an extra cell 4a, the arrangement being such that the cathode ray tubes 2, 2, etc., of one set and the cell 4a of the other set are rendered inoperative alternately with the cell 4 of the first'set and the cathode ray tubes 2a, 2a, etc., of the other set. For this purpose each cathode ray tube 2, 2, etc., is provided with a blocking electrode l4, the cell 4 with a blocking electrode l5, each cathode ray tube 2a, 2a, etc., with a blocking electrode Ma and the cell 40. with a blocking electrode I5a. The blocking electrodes I40. and I5 and the grid of the amplifying tube l1 associated with the cell 4 are connected to a wire l8, while the blocking electrodes l4 and I51; and the grid of the amplifying tube Ila associated with the cell 4a are connected to a wire l6a, the wires l6 and 16a being rendered alternately positive and negative, during periods corresponding to the scanning periods, from a source of potential l8 through a commutator I 9 having semicircular contacts 20 and 2i connected respectively through rings 22 and 23 to the negative and positive terminals of the source I8. When either wire is rendered negative the tubes and cell to which it is connected are rendered inoperative through stoppage of the electron beams in them.

The message currents are continuously supplied to the control electrodes'l3, l3, etc., and I 3a, 13a, etc., of the two sets of cathode ray tubes. However, when the commutator I9 is in the position shown, the beams in the tubes 2a, 2a, etc., are cut ofi, so that recording takes place only on the mosaic 5 of the cell 4, in which the beam is also cut ofi. Consequently translation of charges on the mosaic takes place only in the cell 4a. When the commutator has turned through 180, which it does. in a period equal to the scanning period, the position is reversed. The signals fromthe cells 4 and 4a are supplied to the primary of the same transformer 24', so that there is no break in transmi n.

The receiver of Figure 3 i he reverse of the transmitter in that it has asingle cathode ray tube 25 associated with a number of cells 26, 26', I

etc., equal to the number of cathode ray tubes 2, 2, etc., each containing a mosaic 21, 21'. etc., of mutually isolated photoelectric elements and each having provision for scanning the mosaic with an electron beam. Between the tube 25 and the cells 26, 26', etc., is an appropriate optical system 28 for focusing images of different equal sized portions of the fluorescent screen 29 of the tube on diflerent mosaics 21, 21', etc. Scanning in the tube 25 is synchronized.- with the scanning in the cell 4 and scanning in the cells 26, 26, etc., synchronized with that in the tubes 2, 2', etc., an arrangement similar to that of Figure 2 being used if recording and translation were eflected alternately at the transmitter. The tube 25 has a control electrode 30 to which the incoming signals are applied, so that the intensity of the beam in this tube is varied in accordance with them. The beam will therefore produce on the area of the screen 29, of which an image is focused on a given mosaic of the cells 25, 26', etc., a series of light spots which willproduce on that mosaic, charges corresponding to a given message. As

scanning is simultaneous in the cells 24, 26 and each cell is connected in a separate reproduction circuit, the contemporaneous messages are reproduced separately and simultaneously.

The areas of the mosaic '5 in the transmittin cell 4 occupied by images of the fluorescent screens 3, 3', 3", etc., may be separated from each other by unoccupied areas owing to the fact that the screens themselves are separated by spaces taken up, for example, by the walls of the adjacent cathode ray tubes 2, 2', etc. As a result of these unoccupied areas on the mosaic, the transmission channel would not, with the arrangement of Figure 1, be utilized during the whole time of transmission. If, for example, the unoccupied areas were as wide as the occupied areas, the channel would carry signalling impulses during only half the total time of transmission. In order to utilize the channel during the whole time of transmission, the arrangement illustrated in Figure 4-may be used, in which it.

has been assumed, purely for purposes of example, that the unoccupied areas are equal in width to the occupied areas. In this case, two sets of transmitting devices, each composed of cathode ray tubes and a cell, are associated with each other so that occupied areas on the mosaicof one cell correspond in position to unoccupied areas on the mosaic of the other cell, the signal plates of the two cells being connected to the transmitter in parallel.

In Figures 4 each of the mosaics 51 and 5a is shown as being divided into eight areas of equal height (four cathode ray tubes being assumed),

the occupied area being shaded. While the scan-.

ning beam in cell 41 is traversing the occupied area m, the scanning beam in the cell 4: is traversing the unoccupied area a-.-, and when the beam in the cell 41 is traversing the unoccupied area b1 the beam in cell 41 is traversing he occupied area b2. Thus, the beams in the two cells are never scanning occupied areas simultaneously. Accordingly, during the time that the transmission channel is carrying signals from the cell 41, i. e., during the time that the beam in that cell is traversing the occupied areas a1, 01, e1 and g1, it is carrying no signals from the cell 42, the beam in which is traversing the unoccupied areas do, 02, e: and yr. The signal plates 11 and I: are connected in parallel but, since the charges on the mosaic elements in the unoccupied areas are all equal, no current variation will result from the scanning of these areas by the beam. Consequently, the scanning of an unoccupied area on one mosaic during the scanning of an occupied area on the other mosaic will not interfere with the signals resulting from the latter.

If it is considered desirable to prevent scanning of the unoccupied areas on one mosaic during scanning of the occupied areas on the other mosaic, the system shown in Figure 5 may be used, in which the hot cathode and one deflecting plate giving the horizontal scanning component in one cell are at a different potential from that of the corresponding parts in the other cell. As shown for the purposes of illustration, the potential of the hot cathode 91 and the left hand deflecting plate I I1 in the cell 41 is 50 volts, owing to the drop in resistor 34 while that of the hot cathode 92 and the left hand deflecting plate H2 in the cell 42 is 150 volts, the potential being supplied by a battery 3| and reduced for the cell 41 by a resistance 32. The potential of the accelerating anode I0: is made 100 volts higher than that of the accelerating anode lil1 in order that there may be the same accelerating effect on the beam in both cells. The potential of the right hand deflecting plates in the cells 41 and 42 is gradually raised from 0 to 200 volts. During the rise from 0 to 100 volts, the beam in the cell 41 will traverse the mosaic but the beam in the cell 41 will remain at the left hand side of the mosaic 52 owing to the higher potential of the left hand deflecting plate I I2. During the rise of the right hand deflecting plate 2 from 100 to 200 volts, the beam in the cell 42 will traverse the mosaic 5:, the beam in the cell 41 remainin at the right hand side of the mosaic 51. The variable potential will, at the end of the traverse in the cell 4: suddenly return to zero and both beams will return to the left hand side of their As the same vertical scan- It will be clear from the above description in 30 connection with Figures 4 and 5, that, if the unoccupied areas of the mosaic are greater than the occupied areas, more sets of transmitting devices can be associated with each other in the manner described. Thus, for example, if the unoccupied areas are twice as large as the occupied areas, three sets of transmitting devices will be associated with each other.

When the arrangements of Figures 4 and 5 are used, signalling impulses will be received at the receiver during the whole time of transmission.

If the impulses are applied to a single receiving cathode ray tube, it may be impossible to arrange all the necessary receiving cells in front of it as there will be no unoccupied areas on its fluorescent screen. What may be done in that case is to provide two sets of receiving devices each consisting of a cathode ray tube and a number of cells corresponding to the number of cathode ray tubes in one set at the transmitter, and associate these in a manner similar to that in which the cathode ray tubes and cells are associated at the transmitter, i. e., so that the areas ofthe screen of one cathode ray tube of which images are focused on the mosaics of the cells proper to it, correspond in position with areas of the screen of the other tube of which no images are formed on mosaics. Thus, if parts 51 and 52 in Figure 4 are'considered as being fluorescent screens of two receiving cathode ray tubes, the shaded areas will represent those parts of the screens of which images are focussed on the mosaics of receiving cells. The incoming signals are fed to both cathode ray tubes but are effective only in alternate tubes in alternate equal sized areas corresponding to the areas of the fluorescent screens of the transmitting cathode ray tubes. Thus, while the beam in the first receiving cathode ray tube is traversing an area such as m of which an image is focused on the mosaic of a receiving cell,-

. the images.

them. As soon as this latter beam reaches the next area of which an image is focused on a mosaic and accordingly becomes effective, the beam in the first tube becomes ineflective, as it reaches an area of which no image is for: zed on any mosaic. If desired, the system of Figure 5 may be used at the receiver also, in order to prevent scanning in the areas on the screens of the cathode ray tubes of which images are not focused on the mosaics of receiving cells.

From a consideration of Figure 4 it will be seen that when the mosaic of a transmitting cell is not entirely occupied by images of the fluorescent screens of the transmitting cathode ray tubes and when two sets of transmitting devices are not associated as described in connection with this figure, the speed of translation of the charges on the mosaic into signalling impulses will be greater than a multiple of the recording speed equal to the number of messages. Thus, in the case illustrated, if it is assumed for purposes of illustration that the time taken to record a message on one of the shaded areas in Figure 4 is one second, the scanning beam in the cell must cover the whole mosaic in one second. However, each of the areas of the mosaic occupied by an image of a fluorescent screen of a cathode ray tube is only one-eighth of the total area. Accordingly, the beam in the cell must traverse this fractional area in oneeighth of one second, so that the speed of translation of the charges on that area into signalling impulses is eight times the speed of recording rather than merely four times. Similarly the time for translating electrostatic images of the messages into signalling impulses, when recording and translation are efiected alternately, may be less than that elapsing during the formation of The time of scanning the mosaic, however, will always be equal to that elapsing during the recording of the messages.

When recording and translation are eflected alternately, complete electrostatic images of the messages supplied to the different cathode ray tubes during a single scanning period will be recorded on the mosaic of the transmitting cell and on those of the receiving cells. However, when recording and translation are carried on simultaneously, there will never be complete images of any messages except those supplied to the first and last tubes, e. g., 2 and 2' in Figure 1. In the case of the first tube the message is entirely recorded behind the cell scanning beam in any given scanning period and in the case of the last tube it is entirely recorded ahead of the beam. In the case of the intermediate tubes the earlier parts of the messages are recorded ahead of the beam in a given scanning period and the later parts behind it, the parts recorded ahead being greater and those recorded behind being smaller the near the tube is to the last, e. g., in the case of tube 2 the first third of the message is recorded ahead and the last two-thirds behind the beam, while in the case of the tube 2" the reverse is the case. Those parts of 'messages recorded ahead of the cell scanning beam in any given scanning period are translated by it into signalling impulses in that period, while the parts recorded behind it are not translated until the next period. Thus in the case illustrated of four tubes and one cell, none of the first message in tube 2, the first one-third of the first message in tube 2', the first two-thirds of the first message in tube 2", and all the first message in tube 2" are translated on the first scanning in the cell, while on the second scanning in the cell all the first message in tube 2, the first third of the second message and the last two-thirds of the first message in tube 2', the first twothirds of the second message and the last third of the first message in tube 2", and all the second message in tube 2" are translated. Accordingly, in the case of simultaneous recording and translation, the charges on the mosaic in the transmitting cell although successively translated into signalling impulses are not so in the same order as they are produced, and the charges corresponding to a given massage although translated at a multiple of the recording speed at least equal to the number of messages, e. g., at least at four times the recording speed in the case of Figure 1, are not translated within a time equal to that elapsing during their recording, since the cell beam must scan the mosaic almost twice before all the charges corresponding to a given message in an intermediate cathode ray tube are translated.

Since in every case the relation of scanning speeds in the receiving cathode ray tube and receiving cells is the reverse of the relation of scanning speeds in the transmitting cathode ray tubes and. transmitting cells,the messages will be reproduced exactly as they were recorded, even if they are not sotransmitted.

Although in the case of a large number of contemporaneous messages the signalling impulses will be delivered to the transmission chan-. nel at a very high rate and consequently the frequency of modulation will be high, any possible difficulty in transmission can be avoided by using quasioptical carrier Waves (having a wave length of the order of a few centimetres), whose very high frequency permits very rapid modulation.

I claim:

1. Apparatus for transmitting a number of contemporaneous messages through a single transmission channel, comprising a group of cathode ray tubes, means for controlling the intensity of the electron beam in each tube in accordance with a diiferent message, means for causing the electron beams in all the tubes to scan their respective fluorescent screens simultaneously, a cell, a mosaic within said cell composed of a multiplicity of mutually isolated photoelectric elements, means for focussing images of the fluorescent screens of the cathode ray tubes on the mosaic, and means for scanning the mosaic to produce signalling impulses in a time equal to that elapsing during scanning in the tubes.

2. Apparatus for transmitting a number of contemporaneous messages through a single transmission channel which comprises a mosaic composed of mutually isolated conducting elements capable of electron emission, means for recording the difierent messages simultaneously as distinct series of electrostatic charges on said mosaic, and means for scanning said mosaic in a time equal to that elapsing during the recording of the messages to translate the charges on said mosaic successively into signalling impulses.

3. Apparatus for transmitting a number of contemporaneous messages through a single transmission channel which comprises a mosaic composed of mutually isolated conducting elements capable of electron emission, means for forming simultaneously on said mosaic distinct electrostatic images of the different messages, and means for scanning said mosaic in a time equal to that elapsing during the recording of the messages to translate the charges on said mosaic successively into signalling impulses.

4. Apparatus for transmitting a number of contemporaneous messages through a single transmission channel which comprises a mosaic composed of mutually isolated conducting elements capable of electron emission, means for recording the different messages simultaneously as distinct series of electrostatic charges on said mosaic,vand means comprising an electron stream for scanning said mosaic in a time equal to that elapsing during the recording of the messages to translate the charges on said mosaic successively into signalling impulses.

5. Apparatus for transmitting a 'number of contemporaneous messages through a single transmission channel which comprises a mosaic composed of mutually isolated conducting photoelectric elements capable of electron emission, means for recording the diflerent messages simultaneously as distinct series of electrostatic charges on said mosaic, and means for scanning said mosaic in a time equal to that elapsing during the recording of the'messages to translate the charges on said mosaic successively into signalling impulses.

6. Apparatus for transmitting a number of contemporaneous messages through a single transmission channel which comprises a mosaic composed of mutually isolated conducting photoelectric elements capable bf electron emission, means for recording the diiferent messages simultaneously as distinct series of electrostatic charges on said mosaic, and means comprising an electron stream for scanning said mosaic in a time equal to that elapsing during the recording of the messages to translate the charges on said mosaic successively into signalling impulses.

FRANCOIS CHARLES PIERRE HENRO'I'EAU. 

